The present invention relates to beacon management in wireless communication networks and, more particularly, to a method and apparatus for transmitting a synchronization beacon from a base station to a wireless terminal in a wireless communication network.
In Personal Communications Systems, wireless terminals (WTs), i.e. handsets, typically include a single radio synthesizer tuned to a specific frequency-timeslot (FTs) for receiving a synchronization beacon transmitted from a base station. A single synthesizer is used to make a WT inexpensive and lightweight. However, the radio synthesizer cannot retune to a different FT very quickly and thus prevents the WT from utilizing other FTs for synchronization when moving between and among cells in a wireless network or when the FT of the synchronization beacon changes. Consequently, WTs frequently lose synchronization with the base station when the synchronization beacon FT changes, rendering the WT useless at least temporarily.
Beacon transmission from a base station is dictated by strict standards that require that such transmission last no longer than thirty (30) seconds at any one FT. In addition, a base station cannot continuously use the same FT. In other words, a base station cannot monopolize a FT. As a result, it is not possible to guarantee that a beacon transmitted from a base station will always be on the same FT. Consequently, when a base station is required to change the FT of the beacon, WTs synchronized to the base station at a specific FT will likely lose synchronization.
It is thus desirable to provide a method and apparatus for transmitting a synchronization beacon from a base station that overcomes the above-described shortcomings of the prior art.
In a first embodiment of the present invention, two synchronization beacons are transmitted by a wireless fixed base station (WFB) as part of a time-division signal via a plurality of cell antennae units (CAU) arranged in a cell. The frequency-timeslot (FT) spacing between the two beacons within the time-division signal is controlled by the WFB and is preferably at least two timeslots. The WFB continuously transmits the FT position of the beacons using a MAC layer beacon position message for receipt by a wireless terminal (WT). The beacon position message is decoded by the WT before the WT switched between the beacons to ensure that the WT knows the FT position of the beacon being switched to. The WT is thus always aware of the FT position of both synchronization beacons and can acquire synchronization with the WFB via either one of the beacons. The present invention advantageously permits both dummy bearer signals (signals that do not carry voice) and traffic bearer signals (signals that carry voice) to operate as synchronization beacons. While traffic bearer signals are typically not converted to dummy bearer signals when the communication link between the WFB and WT is terminated, i.e. when communication between the WFB and WT ceases, a traffic bearer signal also operating as a beacon is converted to a dummy bearer signal to prevent synchronization loss between the WT and WFB. In yet another advantageous feature of the present invention, delivery of MAC layer information such, for example, as the beacon position message, is communicated from the WFB to the WT using short page messages. This avoids the priority problems associated with prior art methods, where MAC layer information is transmitted using zero-length paging messages, which are accorded a lower priority than long-length paging messages. This embodiment of the present invention also satisfies the wireless protocol requirement that a beacon not monopolize a FT by xe2x80x9cwinkingxe2x80x9d dummy bearer beacons at a predetermined rate, i.e. either toggling the beacon between on and off states or switching the beacon to a new FT during periods of no voice and/or data traffic being communicated between the WFB and WT in the cell.
In a second embodiment of the present invention, one synchronization beacon is transmitted by each cell in a wireless communication network comprised of a plurality of cells. The network includes a WFB connected to a plurality of CAUs, with each CAU located in a cell. Here too, the beacon can be either a dummy bearer signal or a traffic bearer signal. For dummy bearer signals, transmission from the WFB lasts no longer than thirty (30) seconds. For approximately the first 23.04 seconds, the dummy bearer beacon is considered to be in a living state; for approximately the following 6.40 seconds, the dummy bearer beacon is considered to be in an expired state. The WFB continuously broadcasts the FT position of beacons in the living state, with dummy bearer beacons in the expired state also carrying FT position information for a beacon in the living state in the same cell. The MAC layer beacon position message is transmitted by the WFB in this embodiment in the same manner as described hereinabove for the first embodiment. Since only one synchronization beacon is provided in the network of this embodiment, when a beacon enters the expired state, a new dummy bearer beacon is established at a different FT than the FT position of the expired beacon. The WT is able to synchronize to the new dummy bearer beacon because the beacon position is continuously transmitted by the WFB using the MAC layer beacon position message. In addition, the duration of the beacon in the expired state permits the expired beacon to advertise the position of the new beacon for a predetermined period. Thus, WTs synchronized to the expired beacon will receive the FT position of the new beacon and can resynchronize to that FT. Conversion of a traffic bearer beacon to a dummy bearer beacon only occurs if the WFB has selected a traffic bearer signal as the single beacon in the cell. Finally, before a WT decides to resynchronize to another beacon, i.e. to move from one cell to another, it must first receive and decode the beacon position as received in the MAC layer beacon position message to ensure synchronization with the WFB of the new cell.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.